1) Field of the Invention
The present invention generally relates to in-situ remediation of soil or ground water contaminated by volatile or semi-volatile organic compounds (VOCs or SVOCs) by vacuum extraction, air sparging, or enhanced biodegradation of organic compounds, and in particular to an apparatus for in-situ soil vacuum extraction of the contaminates and the process thereof.
2) Description of the Prior Art
Conventional methods of removing contamination from groundwater and soils include excavating the contaminated subsurface material and pumping contaminated water from subsurface materials to the surface for treatment. Both methods are expensive due to the volume of material which must be removed, treated, and disposed.
In-situ soil vacuum extraction (ISVE) involves the removal of volatile organic compounds (VOC's). Dissolved VOC's are present in the upper, unsaturated "vadose" zone above the groundwater. The dissolved VOC's approach an equilibrium concentration in the soil's pore space according to Henry's Law.
The contaminates can be aliphatic and/or aromatic hydrocarbons, halogenated hydrocarbons, or other volatile organic compounds. Many of these compounds have vapor pressures of between about 1 to 20 mmHg at room temperature. Accordingly, such compounds can be easily volatilized when subjected to a suitable vacuum or air flow.
The design and operation of ISVE remediation systems are based on the movement of gases in a porous media. Thus, a negative air pressure gradient exerted on the soil will induce migration of the subsurface VOC's. The VOC's can then be collected at extraction point(s) and discharged and collected at the surface. Accordingly, an ISVE remediation system must be designed in accordance with site specific subsurface conditions in order to maximize the rate of contaminant removal.
ISVE remediation systems may be either vertical or horizontal wells. Vertical systems become cost competitive with excavation and removal when the vadose zone exceeds about 20 feet in depth. Horizontal systems are more effective where contaminates are very shallow, i.e. less than about 20 feet. However, vertical well systems are prone to plugging.
The vacuum at the wellhead of conventional ISVE remediation systems is directly related to the range of influence (ROI) at the well and, therefore, the rate of removal of VOC's from the site. In addition, it is well known that the spacing of the wells between one another of conventional ISVE remediation systems is critical to the overall performance of the ISVE remediation system. This is due, in part, because the ROI can vary depending on the soil type and depth of groundwater.
For example, Malot and Wood applied a ISVE remediation system at a site in which 15,000 gallons of CCl.sub.4 was spilled in an area where the top of the unconfined aquifer was 300 feet below the surface. Slotted pipes were installed at depths of 75 to 180 feet. A vacuum of 29.9 inches of Hg and a flow rate of 240 CFM was applied. After 90 days the vacuum stabilized at a ROI of 10 feet (Malot, James J. and Wood, P. R., "Low Cost, Site Specific, Total Approach to Decontamination", Conference on Environmental and Public Health Effects of Soils Contaminated with Petroleum Products, University of Massachusetts, Amherst, Mass. Oct. 30-31, 1985). Cited in Cleanup of Releases From Petroleum USRs. 1988. U.S. Environmental Protection Agency. Wash. D.C. April 1988.
Soil venting is also discussed by Connor, R. 1988. Case Study of Soil Venting. Pollution Engineering. 20(7):74-78. and Hutzler, N. 1990. Soil Vapor Extraction of Organic Chemicals. Int'l Symposium on Hazardous Waste Treatment: Treatment of Contaminated Soils, Air & Waste Management Association and U.S. EPA Risk Reduction Laboratory, Cincinnati, Ohio, Feb. 5-8, 1990.
U.S. Pat. Nos. 4,593,760 and Re. 33,102 (U.S. Pat. No. 4,660,639), issued to Visser et al., describe one ISVE remediation system for removing VOC's from the vadose zone. Wells are sunk vertically into the vadose zone. The well casing includes a lower perforated region. VOC's in the vadose layer enter the perforated casing and are pumped to the surface for treatment.
U.S. Pat. No. 4,832,122, issued to Corey et al., also describes another ISVE remediation system for removing VOC's from the vadose zone. Two sets of wells are sunk into the vadose zone. One well injects a fluid into the saturated zone below the plume of contamination. A second well, located above the plume, collects the fluid along with the VOC's from the plume, and pumps it to the surface for treatment.
ISVE remediation systems can have a great degree of success removing VOC constituents exhibiting relatively high vapor pressures and under the proper hydrogeologic settings. In addition, ISVE remediation systems are very cost competitive when compared with other alternatives, including physical removal and disposal in a secure landfill. However, conventional ISVE remediation systems perform poorly for areas having relatively high water tables and/or soils with an extremely high clay content.
An additional approach to ISVE involves the placement of horizontal trenches having a perforated conduit within a porous media and an additional conduit connected to a pump as described in U.S. Pat. No. 5,161,914, issued to Rahn and Chandler. This approach is most effective where contaminants are very shallow, less than 20 feet. However, if the soil excavated for placement of those trenches contains hazardous constituents, the soil itself is hazardous waste and will require disposal or off-site treatment at an approved facility. In addition, some sites requiring remediation have considerable underground obstacles such as utilities or process piping which increase the cost of or prohibit the trench installation
It has therefore become desirable to develop an improved remediation apparatus having an increased working surface area and ROI than vertical wells while decreasing the possibility of the prior art problems involving poor performance in areas having relatively high water tables and minimizing the generation of soil requiring off-site disposal or treatment. In addition, the improved remediation apparatus may be installed at greater depths than conventional trenches while providing a greater ROI than the conventional vertical well approach.